This invention relates to the field of integrated semiconductor lasers and modulators, and more particularly to integrated distributed feedback (DFB) lasers and electroabsorption (EA) modulators with surface-emitting characteristics.
Semiconductor lasers have been the workhorse for fiber optic communication and their role in communication becomes increasingly important as the demand in broadband applications increase rapidly. Overall speaking, there exist no other light sources known to date that can match the size, cost, and performance of semiconductor lasers. However, semiconductor lasers do present a unique set of challenges for optical fiber communication because of their special material and device properties. Asymmetric beam profile and chirping present notable problems. Semiconductor laser research in the last few decades has focused on these problems since semiconductor lasers are the light source of choice for optical communication. Because of the design of semiconductor waveguide and laser cavity, the output beam of a semiconductor laser is elliptical, making efficient laser/fiber coupling a difficult and often costly task. One way to produce a circular output is the surface-emitting laser structure being successfully demonstrated for short wavelength (850 nm) lasers. However, it remains to be a tremendous challenge to make long wavelength (1.3 and 1.55 xcexcm) surface-emitting lasers having a single-mode, symmetric beam profile.
For 1.3 and 1.55 xcexcm lasers, a possible solution is an edge emitting laser with an integrated spot size converter (SSC) that transforms an elliptical beam into a nearly circular beam. However, the extra processing and critical process control make the solution less desirable in cost sensitive systems such as optical LANs. Another critical issue inherent to semiconductor lasers is the much broader linewidth as compared to, for example, solid-state or gas lasers. Under high-speed modulation, the so-called xe2x80x9cchirpingxe2x80x9d effect can significantly broaden the linewidth of a single longitudinal mode laser (e.g., a distributed feedback (DFB) laser) to more than 0.6 nm, measured at 20 dB down from the peak intensity. In the 1.55 xcexcm wavelength regime where the light attenuation in optical fiber is the lowest, such laser linewidth broadening results in dispersion penalty, which is the primary limiting factor for the transmission distance. One solution to the dispersion problem is to externally modulate the laser light instead of directly modulating the laser diode current. Monolithic integration of a semiconductor laser diode with an electro-optic absorption (EA) modulator offers significant advantages than other hybrid approaches. In some advanced devices, the epitaxial layers of the DFB laser and the EA modulator can be formed in a single epitaxial growth run using either the technique of area selective growth or positive wavelength detuning. In the former approach, the growth is performed on a patterned so that different areas of the wafer can have a different growth rate. If the growth rate in the modulator region is slower than the laser region, then the quantum wells in the modulator region will have a blue shift relative to the laser emission, enabling external intensity modulation through the quantum-confined Stark effect. In the latter approach, the active region for the laser and the modulator is identical but the gratings in the laser region have a longer period than the wavelength of the gain peak, also called positive detuning. Since the lasing wavelength for a DFB laser is primarily determined by the grating period, the wavelength that is longer than the natural lasing wavelength matches the modulation wavelength of the optical modulator. Although these devices have been implemented in stand-alone unit, in particular in long haul fiber optic networks, integrating a laser and EA modulator poses a challenge in high volume manufacturing.
Moreover, today""s integrated laser/EA modulator devices are still too expensive for broadband optical data networks such as gigabits Ethernet and for telecommunication networks such as metropolitan area network (MAN) and wide area network (WAN). It would therefore be desirable to provide a low-cost, integrated, externally modulated laser-modulator with a controlled output beam profile. In addition, it would be desirable to include an integrated power monitoring detector to monitor in-situ the extinction ratio of the device. The last property is important to avoid the sensitivity penalty due to the reduction of signal extinction ratio.
The invention is directed to semiconductor laser diode integrated with an external modulator, wherein collimated and/or focused laser radiation is emitted from a major surface. The surface emitting optical beam profile is suitable for fiber coupling. The laser diode can have the structure of a DFB laser having an angle with the axis of the major crystal plane to avoid back coupling of the reflected light into the laser diode. The external modulator can have a similar epitaxial layer structure as the laser diode, but operates in reverse bias. The electrical isolation between the laser and the modulator can be provided by a cleaved facet which creates a small gap between the laser and modulator section, providing excellent electrical isolation with nearly perfect optical coupling between the two sections. The output light from the laser section enters the modulator and is intensity modulated through either Franz-Keldysh effect or quantum confined Stark effect (QCSE) depending on whether the active region is made of bulk crystal or quantum wells. The two cleaved sections are joined to become one unit using either a thick layer of metal or hardened reflowed photoresist. The latter is found to be a more attractive approach because of the simpler process and more desirable properties in terms of adhesion, surface tension, stiffness, and thermal and chemical properties.
At the end of the modulator is an etched facet that directs the beam downward towards the bottom of the substrate, which can include a reflective coating to function as a reflecting mirror. The beam is then reflected upward by the bottom mirror and finally collimated or focused by an integrated microlens. The actual position of the output beam depends on the thickness of the substrate or laser device and the angle of the reflecting facet. The output beam is oriented normal to the major surface with a low and nearly symmetric diffraction angle in both normal axes. To make a collimated output beam, the focal length of the microlens should be designed to be around twice of the substrate thickness, which can be quite easily achieved using the standard microlens fabrication techniques such as resist reflow (refractive lens) and dry etching (diffractive lens). If reflowed resist (e.g. AZ4620) is used as the lens material, it can also be used as the joining material to hold the laser section and the modulator section together.
Embodiments of the invention may include one or more of the following features. The DFB laser section can include a detector that detects the optical power of the laser radiation, in particular laser radiation that is reflected from an end face of the DFB laser section. The modulator section can also include a detector that measures the modulated output power of the surface-emitting laser device which can be, for example, the power averaged over the ON state and the OFF state of the modulator. The extinction ratio of the modulator can then be determined from the ratio of the signals produced by the two detectors. During operation, the bias voltage of the modulator can be adjusted to keep the extinction ratio constant. The DFB laser section can be straight or curved.
The lens that focuses and/or collimates the emitted laser beam can be a refractive or a diffractive optical element. The lens surface and/or the interface between the lens and the major device surface can include an antireflection coating.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.